Chitons have separate sexes and release their gametes in springtime from genital pores located on either side of the body in the pallial cavity. Fertilisation is external and development leads to a trochophore larva, which is plump with yolk and non-feeding or lecithotrophic.

CLICK HERE to see a video of a fly-by along the pallial groove of a chiton, starting from the mouth, moving past the ctenidia, then on to a view of reproductive products issuing from the left gonopore, and ending with a view of fecal products being released from the anus.

Some brief observations on the breeding habits of chiton species in Monterey Bay, California are provided by a researcher from Leland Stanford Jr. University. Two of these are summarised below. Heath 1905 Zoologischer Anzeiger 29: 390.

NOTE unfortunately, most of the dozen or so species listed by the author cannot be traced to extant species now found in California

NOTE now known as Stanford University, but originally endowed by the Stanford family in memory of their son Leland who died in March 1884 of typhoid, at age 15, in Florence Italy while on a tour with his parents

Mopalia muscosa: males release sperm in quiet-water tidepools until interrupted by the returning waves. The sperm drift to neighbouring females, causing them immediately to release eggs from their oviducts

Cryptochiton stelleri: spawning is in February-March following gonadal growth in autumn and early winter. Growth is rapid and by the end of the second year of age, at about 12-17cm in length, they are sexually mature

Research study 1

Black leather chitons Katharina tunicata in Monterey Bay, California build their gonads through winter and spring, and spawn in July (see graph on Left)Gonads remain shrunken from late summer to early winter.

In the same area, Mopalia hindsii spawns in late autumn/winter.The variability of these cycles in the 2 species, especially for M. hindsii, suggests to the authors that the timing of breeding is unlikely to be related to day-length or some other factor invariant from year to year, but rather to be subject to certain local factors. They also note the reciprocal nature of the breeding seasons of the 2 species, Katharina in summer and Mopalia in winter.Giese et al.1959 Biol Bull 117: 81.

NOTE gonad index in this study is calculated as volume of gonad divided by live mass of whole animal x 100. This differs from later studies in which gonad mass is divided by total mass of live animal

NOTE for visual clarity the 2 sets of yearly data are smoothed out for Katharina, but are left visible for Mopalia

Research study 2

Gonad indices for both sexes of the gumboot chiton Cryptochiton stelleri in the Pacific Grove region of California are shown in the accompanying graph. Gonads in both sexes reach maximum sizes in Jan-Feb, with spawning in Mar-May. Spawning is not synchronous in any given population and may continue over a month. The authors consider 3 possible factors regulating gonadal growth in Cryptochiton: light, temperature, and food. With respect to the last, they point out that while gonads are growing in the autumn, seawater temperature is actually declining, and the gonads reach maximum size in winter when food is least available. Tucker & Giese 1962 J Exp Zool 150: 33.

NOTE as in the previous Research Study the authors calculate gonad index as ratio of gonad volume to live mass of animal X 100

Gonads in a chiton vary from 1% gonadal index in a spent animal to 10-15% in a gravid one. Where does the energy for gametogenesis come from? Studies on Katharina tunicata and Mopalia hindsii in Pacific Grove, California indicate that the energy stores responsible are primarily lipids, with a small contribution from glycogen. Lipids comprise up to 29% of the dry mass of some organs in a chiton’s body. The ovaries, for example, store large amounts of lipids, most of which go into the formation of yolk for eggs. In comparison with most other invertebrates in which glycogen represents the main energy reserve, glycogen is in surprisingly low levels in chitons. The authors suggest that glycogen in chitons may be used for fast responses such as locomotion, while lipid may be a reserve for slow processes such as growth of gonads. Protein levels are high in the gonads, as well, and the authors consider the gonad as one of the most important organs of nutrient and energy storage in a chiton’s body. Giese & Araki 1962 J Exp Zool 151: 259; for comparable information on the gumboot chiton Cryptochiton stelleri see Tucker & Giese 1962 J Exp Zool 150: 33.Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

Chiton Mopalia hindsii 1.2X

Research study 4

Reproductive cycles of chitons Mopalia muscosa in the area of Santa Monica Bay, California include gonadal growth through summer, and then 2 or more distinct spawning periods occurring in winter and early spring (see graph). Feeding and growth during winter are minimal. Boolootian 1964 Helgoland Wiss Meer 11: 186.

NOTE data presented are gonadal indices, calculated as live mass of gonad/total body mass including gonad x 100%. The author presents data for 3 areas with males and females combined, only one set of which is shown here. The dotted lines indicate 95% confidence limits

NOTE major foods are various red and green algae, including Gelidium, Gigartina, Polysiphonia, Ectocarpus, Bryopsis, Ulva, and Corallina, and some miscellaneous animal matter including small barnacles, snails, and bivalves. Sometimes considerable sand is also consumed

Research study 5

A 5-yr study on A reproductive cycles in black leather chitons Katharina tunicata at Botanical Beach, British Columbia shows that gonadal growth starts in autumn, coincidental with the seasonal decline in sea temperature. Animals spawn in springtime when sea temperatures begin to rise, correlative with spring plankton blooms.The author finds no evidence that gonadal growth in this geographical area is influenced by food availability. Himmelman 1978 J Exp Mar Biol Ecol 31: 27.

NOTE data sets from another location at Point-no-Point, British Columbia, geographically near to Botanical Beach, are included in the graph (in 1972)

Research study 6

In southern British Columbia the chitons Tonicella lineata and T. insignis build their gonads from summer to mid-winter and spawn in springtime. Although light and temperature are likely influencing factors on the timing of spawning in these species, the author suggests that a strong proximal cue may actually be the presence of phytoplankton in the water. Himmelman 1979 Mar Biol 50: 215.

NOTE the author provides comparable data on gonadal cycle for T. insignis but, as the data are similar to those for T. lineata, they are not included here

NOTE gonada indices are calculated as % live mass of gonad of total live body mass

Research study 7

Gonadal growth prior to spawning of Mopalia species in southern British Columbia is variable but generally occurs between summer and autumn. Spawning, as shown in the accompanying graph for Mopalia ciliata, is usually in springtime coincidental with seasonal return of warmer temperatures and sunlight. This ensures that the newly settled juveniles have abundant supply of algal foods. Himmelman 1980 The Nautilus 94: 39. Photograph courtesy Lovell & Libby Langstroth, California and calphotos

NOTE data presented here are for a population at First Narrows, British Columbia

Chiton Mopalia ciliata 1.5X

Research study 8

Synchronous spawning is necessary in marine invertebrates to ensure maximum fertilisation success. The mechanism controlling this is unknown but must involve some sort of pheromonal stimulation initiated either by males or females, or both. To see if gonadotropin-releasing hormone (GnRH) from a vertebrate would be capable of acting in this way on chitons, researchers at Friday Harbor Laboratories, Washington test GnRHs from a variety of chordates (lamprey, tunicate, chicken). Results show that 2 of these GnRHs, from lampreys and tunicates, cause spawning in a chiton Mopaliasp., but only at high concentration 1mg . liter-1. The authors suggest that a similar molecules may act pheromonally in marine invertebrates to stimulate spawning. Gorbman et al. 2003 Gen Comp Endocrinol 131: 62.Photograph courtesy Ron Long, Simon Fraser University, Burnaby, British Columbia.

NOTE a small peptide molecule produced in the brains of vertebrates. GnRHs function to stimulate release of gonadotropic hormones from the adenohypophysis, which in turn regulate gonadal activity. GnRHs have been reported from several protochordates, including tunicates, but their functions are unclear

NOTE the authors do not identify the species used and note that there may, in fact, have been several Mopalia species involved. The tests are done in early April which, based on spawning times of other chiton species including Mopalia spp. on the west coast, may have been slightly after the peak springtime spawning period